Your lawn mower battery starts your mower without a problem, but after a while, it goes flat. Then, the next time you go to start your mower, the battery doesn’t have enough power to fire up your engine. Sound familiar? Well, the problem might be because a couple of components on your lawn mower aren’t working correctly and not charging the battery as the mower is in use. What are these components? Let’s take a look and find out.
Why Won’t Your Lawn Mower Battery Charge? (The Short Answer)
Lawn mowers typically rely on a stator to generate AC voltage which is then converted into DC voltage through a voltage regulator/rectifier with the aid of several magnets. The DC voltage is then delivered to the battery for charging. A fault in either of these parts will result in your lawn mower battery not charging.
4 Possible Reasons Your Lawn Mower Battery is Not Charging
A mower not charging the battery comes down to 4 common faults within the charging systems. Let’s look over your lawn mower’s electrical systems and see how the charging system works and how it can malfunction.
Damaged Magnets within the Flywheel
A series of magnets inside the flywheel of a lawn mower use a stator to create a charge. The magnets produce AC voltage as they pass the coils of the stator. The voltage created needs to fall within a specific range so that the charge going back into the battery exceeds the voltage going out of the battery as it operates the lawn mower.
For example, if the lawn mower is using 12v and is only receiving 6v through charging, the battery will eventually become flat. So if the magnets are damaged, incorrectly spaced, or missing, the stator will not be able to generate enough AC voltage to charge.
Stator Isn’t Outputting Sufficient Voltage
A stator consists of insulated wires coiled around steel laminations in a circular pattern. As the stator generates AC voltage, the unit heats up and slowly degrades the wires’ insulation. Unfortunately, this process causes short circuits or a broken circuit in the stator, which in turn reduces the AC output.
One thing that I have found that can shorten how long a stator lasts is how hot my lawn mower gets. If you don’t keep the cooling fins of the flywheel clean and the engine oil at the right level and fresh, then your lawn mower can really heat up and damage your charging system.
Voltage Regulator Isn’t Outputting Sufficient Voltage
The voltage regulator/rectifier carries out two jobs. First, it converts AC voltage to DC voltage. Second, it regulates the output voltage so that the output doesn’t exceed the battery’s limits. For example, you want to avoid sending 40v to a 12v battery as this is extremely dangerous.
On the other hand, a bad voltage regulator on your lawn mower that sends 6v to a 12v battery during stator charging will have no charging effect. So, if the regulator inputs and outputs correctly, your battery should have no problem charging.
Voltage Regulator Fuse
Some lawn mower charging systems have a fuse connected to the regulator/rectifier circuit, which sometimes can blow if the circuit draws too much current. So, before grabbing your tools and tester, take a quick look at your lawn mower and see if there is a blown fuse in the charging system. You might be surprised. I’ve seen mowers blowing a fuse when starting and this fuse could be the reason your mower battery is not charging.
Testing a Lawn Mower Charging System
If your lawn mower doesn’t charge its battery, then thankfully, there are 3 quick tests you can do to check the charging system using a multimeter. Let’s take a look at why your lawn mower won’t charge its battery.
Test 1 – Stator Output AC Voltage
To test the output of your stator, you first need to start your lawn mower’s engine and set the throttle to maximum. Then set your multimeter to AC voltage.
Next, connect the multimeter probes to the wires coming from the stator. It doesn’t matter which way around you connect the probes, as stators are not polarity sensitive.
The resulting reading should be anywhere between 12-50v AC on your multimeter. If you don’t get this expected voltage reading, you’ll need to conduct a second test.
Test 2 – Stator Output Resistance
Next, let’s test the resistance through the stator. So, set your multimeter to Ohms. Then, disconnect the stator from the voltage regulator/rectifier. Next, connect the multimeter probe to the two stator wires and observe the multimeter readings.
If you get a reading of 100-200 milliohms through the stator and a lower-than-expected voltage output, you will need to repair the flywheel magnets.
Next, if the stator has a resistance reading of 0 ohms & failed the output test (below 12v output), then the stator is faulty and will need to be replaced.
Then, if the stator has a resistance reading of infinity/open circuit & failed the output test (below 12v output), then the stator is faulty and will need to be replaced.
Finally, if the stator has a reading of 100-200 milliohms and outputs a high output voltage of 12-50v AC, then both the stator and the flywheel magnets are ok, and you’ll then need to test the regulator/rectifier.
Test 3 – Regulator/Rectifier Output Test
Now that you have confirmed that the stator is working correctly and is giving out a high voltage when your lawn mower engine is running, it’s time to test the output of the regulator/rectifier. So, use your multimeter to test the voltage of the battery. If the battery tests below 12.5v DC, then stop and charge the battery.
With your battery fully charged, start your lawn mower’s engine and set the throttle to full. Next, test the voltage of the battery again.
Here is what the voltage reading means.
- 5v or below. The charging system is not working. If the stator is giving a high output, but the battery reading is low, then the regulator/rectifier needs to be replaced.
- 13-15v Normal. The charging system is working correctly, and no further action is required.
- 15v + Overcharging. If your multimeter reads higher than 15v, the voltage regulator/rectifier is faulty and should be replaced.
What to Try If You Find Your Battery Won’t Charge
If one of the three tests fails, you’ll need to conduct a repair to get your lawn mower charging system back up and running again. Here are three fixes that should get the charge back in your battery.
Install Magnets on the Flywheel
To repair the flywheel magnet, you’ll need to gather a few tools and materials. Here’s a list of everything you will need.
Tools & Materials to Install Magnets on the flywheel
- Socket Wrench Set
- Scraper
- Two Part Epoxy Glue
- Clamp
- Torque Wrench
- Multimeter
Steps to Install Magnets on the Flywheel
- Remove the spark plug
- Disconnect both cables from the battery
- Remove the engine cover
- Remove the center bolt securing the flywheel to the driveshaft
- Remove the flywheel key
- Lift off the flywheel
- Turn the flywheel over
- Locate the loose magnet/s
- Clean off old adhesive from the loose magnet & magnet fixing location
- Apply new adhesive to the flywheel
- Position the magnet back in its original position on the flywheel
- Clamp the magnet in place so that it can’t move
- Allow the adhesive to cure
- Remove the clamp
- Install the flywheel back on the drive shaft
- Install the flywheel key
- Install the flywheel nut
- Torque the flywheel nut to the manufacturer’s setting
- Reinstall the engine cover
- Reconnect both cables to the battery
- Reconnect the spark plug
- Run the engine and test the output voltage (test 1 – stator output voltage) of the stator to confirm the repair is successful
Replacing the Lawn Mower Stator
When replacing your lawn mower’s stator, you’ll first need to get hold of a stator that matches your lawn mower’s specific engine. Once you have your replacement stator, these are the tools you’ll need for the installation.
Tools & Materials to Replace a Lawn Mower Stator
- Socket Wrench Set
- Torque Wrench
- Multimeter
- Engine Specific Stator
Steps to Install a Lawn Mower Stator
- Remove the spark plug
- Disconnect both cables from the battery
- Remove the engine cover
- Remove the center bolt securing the flywheel to the driveshaft
- Remove the flywheel key
- Lift off the flywheel
- Disconnect the stator connector
- Remove the stator mounting bolts
- Lifts off the old stator
- Position & align the replacement stator
- Route the stator cables
- Connect the stator connection
- Secure the stator in place
- Install the flywheel back on the drive shaft
- Install the flywheel key
- Install the flywheel nut
- Torque the flywheel nut to the manufacturer’s setting
- Reinstall the engine cover
- Reconnect both cables to the battery
- Reconnect the spark plug
- Run the engine and test the output voltage (test 1 – stator output voltage) of the stator to confirm the repair is successful
Replacing the Regulator/Rectifier Module
The last repair you might need to carry out if your lawn mower won’t charge its battery is replacing the regulator/rectifier. Here are all the tools and parts you’ll need to have on hand to get started.
Tools & Parts to Replace a Lawn Mower Regulator/Rectifier
- Socket Wrench Set
- Multimeter
- Engine Specific Regulator/Rectifier
Steps to Replace a Lawn Mower Regulator/Rectifier
- Remove the spark plug
- Disconnect both cables from the battery
- Note the position of the cables attached to the regulator/rectifier unit
- Remove the stator connector from the regulator/rectifier unit
- Remove the mounting bolts of the regulator/rectifier unit including the ground wire attached to one of the fixings
- Lift the old regulator/rectifier unit from the lawn mower
- Position the new regulator/rectifier unit
- Install the mounting bolts including the ground wire
- Reconnect the connector/cables from the stator
- Reconnect both cables to the battery
- Reconnect the spark plug
- Start the engine and retest the battery using test-3
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